In each of segmented Doppler modes, a pulse Doppler mode (D mode) of 20 ms, for example and a B mode of 17 ms, for example are alternately executed as shown in FIG. 6(a). Therefore, Doppler sounds are rendered intermittent. As a result, the Doppler sounds are hard to be used in diagnosis if they are kept intact.
It has therefore been proposed a technique wherein Doppler signals produced during D mode periods are joined to one another and Doppler signals produced during B mode periods immediately after the D modes periods are created to thereby make Doppler sounds continuous as shown in FIG. 6(b). If joints between adjacent waveforms are discontinuous, then trickling sounds are heard so as to offend the ear. Therefore, windows are effected on the joints so as to smooth the Doppler signals.
As another prior art, there has been known one in which signal correlation characteristics are determined from Doppler signals produced during D mode periods, a coefficient of an FIR filter (Finite Impulse Repeat filter) is decided and random data is inputted to the FIR filter to thereby create Doppler signals to be used during B mode periods immediately after the D mode periods and to smoothly join the created Doppler signals to the Doppler signals before and after the created Doppler signals under the action of windows so that Doppler sounds are rendered consecutive.
As a further prior art, there has been known one in which power spectrums of Doppler signals produced during D mode periods existing before and after B mode periods are determined, power spectrums for the B mode periods are determined by linear interpolation of both power spectrums and random phases are given to the power spectrums to thereby create Doppler signals to be used during the B mode periods and to smoothly join the created Doppler signals to the Doppler signals before and after the created Doppler signals under the action of windows so that Doppler sounds are rendered continuous.
The above prior arts are accompanied by problems that although the Doppler signals to be used during the B mode periods are created and thereafter smoothly joined to the Doppler signals existing before and after the created Doppler signals under the execution of the windows, a change in sound quality occurs when the windows are effected. Further, a problem arises that the decision oft the FIR filter coefficient and the determination of the linear interpolation of the power spectrums provide a complex structure.